Diethylenetriaminepentaacetic acid gadolinate (hereinafter abbreviated as "DTPA-Gd")JP-A 58-29718! which is widely used as a nuclear magnetic resonance diagnostic agent is a typical example of monometal complexes and the effectiveness thereof as a diagnostic imaging agent in the brain or the spinal cord region has almost been established. However, the relaxivity of DTPA-Gd is lower than that of Gd itself, because Gd is chelated with DTPA. Therefore, there is a need for compensating for the lowered relaxivity with the increased dosage or the like. In addition, DTPA-Gd is rapidly excreted into the urine after administration Hiroki Yoshikawa et al., Gazoshindan, 6, 959-969, 1986!, and this is very disadvantageous for imaging of several parts of the body by reflecting them in blood stream (blood vessel distribution, blood stream distribution, distribution volume, permeation and the like in a lesion) with a single injection of the pharmaceutical. Further, such the rapid excretion also makes distribution properties of DTPA-Gd extremely disadvantageous.
In order to solve these problems, a diagnostic imaging agent obtained by chemically bonding a bifunctional ligand to an amino oligosaccharide and coordinating a metal ion thereto via this bifunctional ligand JP-A 5-97712!, and a diagnostic imaging agent obtained by chemically bonding at least one bifunctional ligand to a dialdehyde-saccharide wherein a plurality of D-glucose molecules are bonded and at least one constituent monosaccharide among them is oxidation-cleaved JP-A 5-25059! are developed. However, it was found that, when a raw material saccharide, an important component of these compounds, is left or reacted for a longer period of time, the intramolecular or intermolecular bonding of the saccharide, or peroxidation of the saccharide by an oxidizing agent occurs in an aqueous solution unless a reducing end of a sugar chain has been treated.